The in vitro transport of L-lactate across rumen epithelium was found to be a passive, mass-action regulated diffusion, and no saturable transport mechanisms were involved at concentrations from 2-20 mmol/L. During this increase in L-lactate concentrations the epithelial resistance and potential difference increased significantly, indicating an epithelial change towards a more tight epithelium. The net sodium transport was significantly reduced by 45% at 20 mmol/L lactate as compared to lactate free conditions.